In the past it has been customary to deliver charges of molten glass by utilizing a conventional tapered well with a cylindrical gobbing needle. Normally, such conventional tapered wells are relatively shallow, with the depth being comparable to the diameter, so that the range of stroke lengths of the gobbing needle are rather limited. Further, since the well is of a tapered configuration, the cross section is not uniform, and accordingly a non-uniform needle speed is required in order to produce a uniform flow rate at the orifice.
The conventional tapered well delivery system has not been completely satisfactory for all purposes due to the lack of adequate control obtainable between the tapered walls of the well and the gobbing needle. The control coupling obtainable between the needle and the walls of the tapered well is limited to a line coupling between the needle and the closest portion of the tapered well, while the needle is in its lowermost position. When the needle is raised upwardly, the control coupling is virtually nonexistent since the upwardly diverging tapered walls of the well produce a relatively large glass-flow opening between the needle and the well, which precludes an effective flow-control coupling therebetween. Accordingly, although some degree of control is obtainable with the tapered well configuration, the control is dependent upon needle position within the well and the utilization of relatively high viscosity glasses.
A further disadvantage encountered with the conventional tapered well resides in the fact that it is very difficult to prescribe the required needle velocities in order to obtain zero flow at the orifice when changing gob volume. That is, when the mean position of the needle in the well is raised or lowered to increase or decrease the gob volume respectively, it is necessary to change the rate of needle stroke in order to provide zero flow and prevent suckup at the orifice intermediate the gobbing cycle. The tapered well gobbing system has a further disadvantage in that it is practically impossible to separate gob weight control from suckup control, since by the very nature of the tapered well construction, one influences the other, and as a result a completely new cam configuration, prescribed by trial and error, is required every time a change in gob volume is made, in order to obtain zero flow at the orifice.
U.S. Pat. No. 3,620,705 to Dockerty sets forth a method and apparatus for feeding molten glass which overcame the problems heretofore enumerated with respect to the conventional tapered well feeders, by utilizing a straight well feeder with an elongated cylindrical needle. However, the elongated needle did not operate wholly within the confines of the well and relied upon a variable coupling between the needle and sidewalls of the well dependent upon its position therewithin, and accordingly the flow of molten glass through the discharge orifice was not only a function of the needle speed, but also a function of its position within the well. Therefore, the relationship defining the glass flow through the orifice became rather complex, resulting in the necessity of utilizing very complicated computer programs to define the required cam contour for operating such needle.